The genetic control of flowering in higher plants in general, and strawberry in particular, has remained unsolved despite recent advances in the fields of plant breeding and molecular biology. It is well known that the induction of flowering in higher plants involves the interaction of genetic factors and environmental cues. The family Rosaceae, of which Fragaria ananassa (cultivated strawberry) is a member, is an excellent example. Cultivated strawberry, an octaploid resulting from a cross between E. chiloensis and F. virginiana, grows as a rossette. The stem is often referred to as a crown, where growing parts are transformed from vegetative to reproductive growth buds. The reproductive buds develop a cluster of inflorescences carried on a common peduncle.
The environmental conditions promoting either vegetative or reproductive development in strawberry are usually antagonistic (Darrow, 1966, The Strawberry: History, Breeding and Physiology, Ch.19, pp.314-334; Holt, Rinehart and Winston; New York, Chicago, San Francisco; Guttridge, C. G., Fragaria x Ananassa, In: CRC Handbook of Flowering, Vol. III, (Halevi A., ed.), CRC Press (1985), pp. 16-33. Two environmental factors controlling the induction of flowering in strawberry are light (photoperiodism) and temperature (thermoperiodism). Cultivated lines of strawberry have been classified as either short-day (Wareing and Phillips, 1978, The Control of Growth and Differentiation in Plants, p. 202, Pergamon Press), day-neutral (Downs and Pringer, 1955, Proc. Amer. Soc. Hort. Sci. 66:234-236), facultative short-day (Guttridge, 1969, Fragaria The Induction of Flowering. Ch. X, ed. Evans, L. T., Canberra: MacMillan, pp. 247-267) or long-day (Guttridge, supra). The above terminology describes the day length in which induction of floral bud initiation occurs and, hence, a switch from a vegetative to a reproductive phase of development. Therefore, in general terms, a short-day plant requires one to numerous light:dark cycles in which day length is becoming increasingly shorter (e.g., from Jun. 21 to Dec. 21). Conversely, a long-day plant requires one to numerous light:dark cycles in which the day length is becoming increasingly longer (e.g., from Dec. 21-Jun. 21). Therefore, a short-day plant may initiate flower bud primordia when the day length is still relatively long, yet becoming shorter day by day. A day-neutral plant is not markedly affected by the light regime. The classification of some strawberry lines as facultative short-day refers to the fact that these lines may flower even under continuous light provided that the average temperature is about 10.degree. C. (Guttridge, 1969, Fragaria In: The Induction of Flowering. Ch. X, ed. Evans, L. T., Canberra: MacMillan).
The transition from the vegetative to the reproductive phase in Fragaria is of immense economic importance. The commercial strawberry supply is limited from November to February despite the wide geographical dispersion of commercial strawberry plantings. Therefore, it would be advantageous to identify strawberry types that initiate floral bud differentiation earlier than those presently available to the commercial grower.